In recent years, due to the widespread use of DVDs and CDs, semiconductor lasers are used in various fields, and reductions in their manufacturing cost and lead time are strongly required.
A burying ridge-type stripe structure that is generally used for a semiconductor laser is shown in FIG. 6 (see JP6(1994)-237038 A). This structure is formed by the method described below.
First, a buffer layer 202, a first conductivity type cladding layer 203, a guide layer 204, an active layer 205, a guide layer 206, a second conductivity-type cladding layer 207, a hetero buffer layer 208 and a cap layer 209 are grown epitaxially on a first conductivity-type substrate 201 using an organometallic vapor phase growing method (hereinafter, called MOCVD method).
An insulating layer (not shown in the figure) is formed on the cap layer 209 and is formed into a stripe pattern, and then the second conductivity-type cladding layer 207 is formed into a ridge shape by etching with the stripe pattern as a mask. Subsequently, a first conductivity-type current blocking layer 210 is grown selectively using the MOCVD method. After removing the stripe pattern, a second conductivity-type contact layer 211 is grown using the MOCVD method. Moreover, a n-side electrode 212 and a p-side electrode 213 respectively are formed.
With this structure, the MOCVD process is carried out three times in the manufacturing process, thus it is hard to reduce the manufacturing cost.
Next, a ridge-type stripe structured semiconductor laser having a dielectric film as a current blocking layer is illustrated in FIG. 7A (see JP11(1999)-186650 A). In the process of forming this structure, a buffer layer 302, a first conductivity-type cladding layer 303, a guide layer 304, an active layer 305, a guide layer 306, a second conductivity-type cladding layer 307, a hetero buffer layer 308 and a cap layer 309 are grown epitaxially on a first conductivity-type substrate 301 using the MOCVD method. An insulating layer (not shown in the figure) is formed on the cap layer 309 and is formed into a stripe pattern, thereafter, the second conductivity-type cladding layer 307 is formed into a ridge shape by etching with the stripe pattern as a mask. Moreover, a current blocking layer 310 made of a dielectric film is provided to both sides of the ridge. Furthermore, a n-side electrode 311 and a p-side electrode 312 respectively are formed.
Since the ridge-type stripe structure using the dielectric film requires the MOCVD process to be carried out only once, the manufacturing cost can be reduced compared to that in the example of FIG. 6. In addition, by virtue of the reduction in the number of growing steps, lead time in manufacturing processes also can be reduced. Particularly in a monolithic dual wavelength laser, steps of crystal growth and manufacturing processes are more complicated, thus it is considered that the cost and the manufacturing lead time can be reduced dramatically by adopting the structure of FIG. 7A.
The laser having the ridge-type stripe structure shown in FIG. 7A, which is manufactured using the epitaxial growth only once, uses as the current blocking layer a dielectric film, such as a silicon oxide film (hereinafter, called SiO2) or a silicon nitride film (hereinafter, called SiN), which has a considerably small refractive index in comparison with that of an AlGaAs based or AlGaInP based semiconductor layer. The refractive indices with respect to light with a wavelength of 650 nm are, for example, AlGaAs=3.1 to 4.1 and AlGaInP=3.2 to 3.6; while SiO2=1.5 and SiN=2.0.
Accordingly, the difference in a refractive index between the current blocking layer and the semiconductor layer composing the ridge portion and the cladding layer becomes larger. Therefore, in order to adjust a flare angle of outgoing light, it is necessary to increase a film thickness H of the second conductivity-type cladding layer 307 outside the ridge portion, which is shown in FIG. 7A, so as to strengthen the light confinement. According to this, as shown in FIG. 7B, an ineffective component 314 of a current to be injected to the active layer 305, which leaks out of the light-emission region 313, increases, thus causing a problem of the increment of an operation current. The increment of the operation current has a risk of degrading the temperature properties of the semiconductor laser element and degrading significantly the reliability at a high temperature.